1. Field of the Invention
The present invention relates to,an optical device and the method of manufacturing the optical device having optical waveguides which are used in, for example, local optical networks, and particularly relates to multi-mode type optical star couplers having optical waveguides (hereinafter referred to as "waveguide type multi-mode star coupler"). More particularly, the present invention relates to waveguide type multiple branched optical star couplers to be connected to graded index type multi-mode optical fibers, each fiber being 50 microns in core diameter and 125 microns in clad diameter, greatly improving the device's performance.
2. Description of the Prior Art
A multi-mode optical fiber has the advantage of high coupling efficiency with light sources, because of its large numerical aperture, compared to a single mode fiber, generally used in long distance communications. Another advantage of a multi-mode optical fiber because of its large core diameter is that the optical axes between different optical fibers or between the fiber and an optical device can be easily adjusted.
An example of a multi-mode optical fiber is a graded index type multi-mode optical fiber, 50 microns in core diameter and 125 microns in clad diameter (hereinafter referred to as the "50/125 GI optical fiber"). This type of multi-mode optical fiber is widely used, for example, in optical local networks and optical instrumentation control systems in the industrial field. It is expected that the 50/125 GI optical fibers will be further popularized and used more widely in the future because of the introduction of optical networks into offices and production sites, and because of the development of such optical networks.
An optical star coupler is an important optical device located in an optical network used to directly combine and distribute light signals. By using an optical star coupler, an optical network, the creation of an optical network used to combine and distribute light signals may be realized. Such an optical network is called a passive optical network, economically superior to an conventional active optical network, which requires the use of expensive converters to change light into electricity or electricity into light. In order to establish economically superior passive optical networks with star couplers, it is indispensable to enhance the performance of multiple branched optical star couplers, that is, multi-mode optical star couplers having many ports, frequently used in optical local networks, and to supply inexpensive products.
Manufacturing a waveguide type optical star coupler by connecting optical fibers to optical circuits (optical waveguides) for splitting and mixing light signals has two advantages: 1) multiple branched optical circuits can be easily obtained, because optical circuits are designed to have a high degree of freedom, compared to optical fiber fused type optical star couplers, which are manufactured by binding optical fibers by fusion; 2) multiple branched optical circuits are producible in large quantities.
The main performance indexes of optical star couplers are loss variance (distribution ratio) and optical loss.
Each piece of equipment or device connected to another piece of equipment or device through optical star couplers usually receives light signals at one of its receiving portions, transmitted from several other pieces of equipment or devices. Therefore, it is very important in designing the system that the receiving piece of equipment or device not be greatly affected by differences in the transmitting piece of equipment or device, and that the power of light signal received be approximately constant. In other words, light should be equally split regardless of which port the light enters the optical star coupler. If the loss variance of the optical star coupler greatly varied depending on the input ports, it would become necessary to expand the receiving sensitivity range of the receiving portion and to adjust the receiving sensitivity in accordance with the transmitter, and it would thus become difficult to design the system. As a result, the system itself would become expensive.
If the loss of the optical star coupler were small, the light receiving power of the receiving portion could be raised and the distance of the light signal transmission by optical fibers could be extended. As a result, it would become easier to design the system, and the system would be more economically efficient.
Especially, the most important issue for the future concerning the performance of multiple branched star couplers widely used in optical local networks is to improve the loss variance. The expression "multiple branched" means that the number of branches is, for example, 8 or more. An optical circuit of a waveguide type multi-mode optical star coupler is usually composed of a main optical waveguide and branched optical waveguides, which branch off respectively on the input side and on the output side of the main optical waveguide. The main optical waveguide functions to mix the mode of light signals entering the branched optical waveguides and to distribute the power of light signal to each branched optical waveguide on the output side through the main optical waveguide. Accordingly, the issue concerning the loss variance of the waveguide type multi-mode optical star coupler is to discover the optical circuit construction that enables effective mode mixing at the main optical waveguide and equal distribution of the mixed light.
A conventional method is described in, for example, Japanese Patent Laid-Open SHO No. 56-140305, which is to attain effective mode mixing through use of a the waveguide type multi-mode optical star coupler. According to this method, it is suggested that the width of the main optical waveguide be made wider at the center and narrower at its both ends, that is, to construct the main optical waveguide with a taper construction in order to heighten the mixing effect in the main optical waveguide.
Conventionally, a circuit with optical waveguides, each having a constant width, i.e., diameter, from the beginning side end (a joint portion with a main optical waveguide) to the terminal side end (a joint portion with an optical fiber), has been used as the branched optical waveguide of a waveguide type multi-mode optical star coupler. However, the problem with this type of optical device is that light cannot be distributed equally from the outside ports existing along the peripheries of the central portion of the device on the light receiving side, to the outside ports on the light ejecting side.
As described above, the greatest problem of the conventional waveguide type multiple branched multi-mode optical star couplers is inconstant loss variance. Therefore, it is indispensable to make some improvements by discovering an optical circuit construction which will not cause the above described light polarization.
The objective of the present invention is to provide optical devices which can effectively mix a plurality of entering light signals and can distribute light signals equally without polarization.
Another objective of the present invention is to provide optical devices for which loss of light signals is reduced.
A further objective of the present invention is to provide passive optical networks which are highly flexibile and economical.
Another objective of the present invention is to provide a method of manufacturing such optical devices.